LMX2364SLEX datasheet(37/39 Pages) NSC

37 / 39 page

Supplemental Information (Continued)

3.5 RF PLL FASTLOCK REFERENCE TABLE

The table below shows most of the trade offs involved in

choosing a steady-state charge pump current (RF_CP), the

FastLock charge pump current (RF_CPF[1:0]), and the

Cycle Slip Reduction Factor CSR.

Parameter

Advantages to Choosing Smaller

Advantages to Choosing Larger

RF_CP

1. Allows capacitors in loop filter to be smaller

values making it easier to find physically

smaller components and components with

better dielectric properties.

Phase noise, especially within the loop

bandwidth of the system will be slightly worse

for lower charge pump currents. If the charge

pump gain is at least 4 mA, most of the

phase noise benefit will be realized.

2. Allows a larger loop bandwidth multiplier for

FastLock, or a higher cycle slip reduction

factor.

RF_CPF

The only reason not to always choose this to

16 mA is to make it such that no FastLock

resistor is required for FastLock. For 3rd and

4th order filters, it is not possible to keep the

filter perfectly optimized by simply switching in

a resistor for FastLock.

This allows the maximum possible benefit for

FastLock.

RF_CSR

Do not choose this any larger than necessary

to eliminate cycle slipping. Keeping this small

allows a larger loop bandwidth multiplier for

FastLock.

This will eliminate cycle slips better.

3.6 CAPACITOR DIELECTRIC CONSIDERATIONS

The LMX2364 has a high fractional modulus and high

charge pump gain for the lowest possible phase noise. One

consideration is that the reduced N value and higher charge

pump may cause the capacitors in the loop filter to become

larger in value. For larger capacitor values, it is common to

have a trade-off between capacitor dielectric quality and

physical size. Using film capacitors or NP0/CG0 capacitors

yields the best possible lock times, where as using X7R or

Z5R capacitors can increase lock time by 0 – 500%. In

general, designs with higher comparison frequencies tend to

be less succeptible to degradations in lock time due to

capacitor dielectric effects. Capacitor dielectrics have very

little impact on phase noise or spurs. Although the use of

lesser quality dielectric capacitors may be unavoidable in

many circumstances, allowing a larger footprint for the loop

filter capacitors, using a lower charge pump current, and

reducing the fractional modulus are all ways to reduce ca-

pacitor values.

LMX2364SLEX Datasheet(PDF) 37 Page - National Semiconductor (TI)

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